Add citation info in README + a CITATION.cff

CITATION.cff

@@ -0,0 +1,15 @@
+cff-version: 1.2.0
+preferred-citation:
+  type: article
+  authors:
+  - family-names: "Qian"
+    given-names: "Yu-Kun"
+    orcid: "https://orcid.org/0000-0001-5660-7619"
+  title: "xinvert: A Python package for inversion problems in geophysical fluid dynamics"
+  journal: "Journal of Open Source Software"
+  doi: "10.21105/joss.05510"
+  volume: 8
+  issue: 89
+  start: 5510
+  year: 2023
+

README.md

@@ -79,4 +79,25 @@ psi = animate_iteration(invert_Poisson, vor, iParams=iParams,
                         loop_per_frame=1, max_frames=40)
 ```
 
-See the animation at the top.
+See the animation at the top.
+
+
+
+## 5 Cite
+
+If you use the package in research, teaching, or other activities, we would be grateful
+if you mention `xinvert` and cite our paper in JOSS:
+
+```bibtex
+@article{Qian2023,
+    doi = {10.21105/joss.05510},
+    url = {https://doi.org/10.21105/joss.05510},
+    year = {2023},
+    publisher = {The Open Journal},
+    volume = {8},
+    number = {89},
+    pages = {5510},
+    author = {Yu-Kun Qian},
+    title = {xinvert: A Python package for inversion problems in geophysical fluid dynamics}, journal = {Journal of Open Source Software}
+}
+```